Abstract Postural tachycardia syndrome (POTS) is a relatively common condition affecting mostly otherwise healthy young women. It is the cause of significant disability and an impairment in quality of life of a magnitude comparable to patients with chronic obstructive pulmonary disease or congestive heart failure. It is characterized by sympathetic activation but the underlying pathophysiology is heterogeneous. In most patients sympathetic activation is likely an appropriate compensatory response to deconditioning, partial neuropathy or hypovolemia. Our preliminary studies, however, show that a very high salt diet administered under controlled conditions in a metabolic unit does not resolve the volume deficits or sympathetic activation in POTS patients. On the other hand, we have identified a subset of patients with high resting supine central sympathetic outflow, as determined by muscle sympathetic nerve activity (MSNA) above the upper 95% confidence interval for the group. This ?primary sympathetic? (psPOTS) subset is associated with a paradoxical increase in upright blood pressure and pressor responses to Valsalva, and appear to improve clinically on central sympatholytics. Thus, our overarching hypothesis is that there is a subset of POTS patients with a central sympathetic activation as the primary pathophysiology. We propose to test this hypothesis in a double blind, placebo- controlled, randomized study using the central sympatholytic moxonidine. If our hypothesis is true, sympathetic inhibition will improve orthostatic symptoms (Specific Aim 1), blood volume (Specific Aim 2) and inflammation (Specific Aim 3) in psPOTS. We would expect worsening of all these parameters if sympathetic activation is compensatory in nature. Moxonidine was selected for this proof-of-concept study because it is an effective sympatholytic imidazoline agonist that has less sedative effect compared to older agents. We will also determine the effect of a high salt diet, often used in the treatment of POTS to improve plasma volume and reflexly reduce sympathetic activation. It is important to determine the effect of salt in psPOTS because a high salt diet may paradoxically increase sympathetic activity and induce inflammation in susceptible (salt sensitive) individuals. We have designed a study that will allow us to determine the effect of a high salt diet alone, or in combination with moxonidine, in psPOTS. We believe the proposed studies will advance this field and ultimately help our patients.